Free Automotive Guide: Exhaust Gas Recirculation

Modern automobiles and light trucks have been equipped with exhaust gas recirculation (EGR) valves since the early 1980s. The purpose of the EGR valve, as the name implies, is to recirculate unburned exhaust fuel back through the engine’s induction system, in order for it to be reused. This process theoretically reduces engine noxious oxide emission and increases fuel efficiency, at the same time. Obviously different automakers use varying methods of implementing the EGR valve. We will examine basic EGR valve operation as well as a few of the most common methods for actuation in this article.

The EGR valve consists of two parts. The first part is a diaphragm, which is attached to an actuator rod. Both are located inside of a sealed housing. The second part is made up of a heat resistant housing with two openings; one is an inlet port and the other an outlet port. The inlet port is blocked using a pendulum valve, which is activated using the vacuum operated diaphragm. The inlet port is typically positioned over a small exhaust opening near the intake. Sometimes flexible EGR tubes are used in order to route exhaust from the manifold, which is located lower on the engine, to the induction system, which is located higher on the engine. At any rate, the inlet port of the EGR valve must be fastened securely to this exhaust outlet, with a gasket type seal in place. The second EGR port, called an outlet port, is fastened in like manner over an air induction opening. This opening will ordinarily be located in the intake or throttle body assembly. When the diaphragm pulls the pendulum valve away from the inlet opening of the EGR, pressurized exhaust gases are allowed to flow through the valve and into the engine, where they will be burned and reenter the exhaust system. When the vacuum signal is no longer received by the EGR the pendulum valve will close, with the aid of a strong spring, and no more exhaust gases will be allowed to flow through until the signal is received again. This process is duplicated nearly every time that the throttle is opened, assuming that all other parameters for EGR operation have been met.

If the engine has not reached an RPM level that is significantly above idle, the exhaust gases which enter the intake will create a lean condition causing the engine to run very roughly and/or stall. It is imperative that the EGR valve be opened only under appropriate acceleration, then immediately closed as the throttle is closed.

Two basic methods are used for EGR valve actuation. Most passenger cars and light trucks use engine vacuum to operate the EGR, which allows exhaust gases to enter the intake or induction system. Having said that, there are quite a few differing methods of using vacuum to actuate the EGR valve. The earliest EGR valves, used on carbureted engines, were operated by using ported vacuum. Ported vacuum exists only when the throttle is opened a certain degree. This allows engine vacuum to reach an outlet in the base of the carburetor. A small rubber vacuum hose is attached from the outlet to the EGR valve. When adequate vacuum reaches the EGR valve, the diaphragm is “sucked” upward, pulling the pendulum valve open and allowing pressurized exhaust gases to flow through the valve and into the engine where it will be burned as fuel and reenter the exhaust system. Early throttle body fuel injection systems also used this system of ported vacuum EGR valve actuation.

Several varying methods of vacuum delivery are now in place in modern OBD-II equipped automobiles. The most common method utilizes vacuum control solenoids which open and close on command from the powertrain control module (PCM). A constant supply of vacuum is routed to the electrically controlled solenoid valves, along with a battery voltage signal wire and a ground wire. The PCM gathers data which determines if and when predetermined requirements have been met for EGR system function to be enabled. Determining factors for EGR system engagement include throttle position, engine load percentage, engine RPM rate, and vehicle speed. When the necessary requirements are met for EGR operation, the PCM applies a ground signal to the EGR control solenoid. When this 12-volt circuit is completed, the solenoid is opened allowing vacuum to reach the EGR valve, opening the EGR pendulum and allowing exhaust gases to flow through it and into the intake where it will be burned and will reenter the exhaust system.

Electronic or linear EGR valves are also used on many automobiles. Instead of using vacuum to actuate the EGR valve, small solenoids are integrated into the EGR valve. These solenoids are supplied a constant 12-volt signal. The circuit is completed with a ground signal, supplied at the appropriate time, from the PCM. When the circuit is completed the electronic solenoids open the valve, allowing pressurized exhaust gases to enter the engine where they will be burned as fuel and reenter the exhaust system.

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S.M. Darby

I am a freelance author with over 25 years of experience as a professional, ASE certified automotive technician and shop owner, muscle car enthusiast, avid street racer, and classic car restoration specialist.